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What is the reason for the poor low-speed characteristics of sweptback wings?
I know there are advantages of swept-back wings delaying shock-wave allowing a aircraft to fly faster. However, what are the disadvantages.
I know one of them is that they have very poor low speed characteristics, however I do not know the reason. Another disadvantage will be on the F-100 Super Sabre Dance, do not understand these either.
Can any kind soul explain the disadvantages?

Related: http://aviation.stackexchange.com/questions/1726/why-do-some-military-aircraft-use-variable-sweep-wings

I don't think this is a duplicate. The related questions, http://aviation.stackexchange.com/questions/1726/why-do-some-military-aircraft-use-variable-sweep-wings, http://aviation.stackexchange.com/questions/9287/how-does-wing-sweep-increase-aircraft-stability and http://aviation.stackexchange.com/questions/15349/optimal-wing-sweep-angle-for-passenger-airliner mainly list advantages of swept wings, but neither of them enumerates the disadvantages, at least not all of them.

I don't think this is a dupe either. This is a reasonably specific question about an aspect of swept wings not directly addressed by those other questions. The low speed characteristics and other disadvantages might have been mentioned in an answer to those other questions, but the questions themselves were about something else.

Disadvantages of wing sweep

Lift curve slope is reduced by the cosine of the quarter chord sweep angle. This means more angle of attack for takeoff, which requires a longer take-off run and a longer landing gear to avoid a tail strike on rotation.
If you rotate the airplane, the tips of backward swept wings come down when the aircraft rotates for take-off. This also might drive the requirement for a longer landing gear.
Bending moments in the wing will become torsion moments when you change the sweep angle. And you will need to change it at least in the wing root where the wing connects to the fuselage. This translates into a heavier structure.
If your sweep angle and aspect ratio both are large enough, the wing will show nasty stall characteristics. The boundary layer is swept towards the tips and causes earlier separation when the wing stalls, and the airplane will pitch up or roll uncontrollably. Wing fences help, but cannot completely remedy this.
Wing sweep causes a yaw-induced rolling moment, so less dihedral is needed. On high wing airplanes this requires to use anhedral. Unfortunately, this rolling moment varies with lift coefficient, so your yaw-induced rolling moment on a swept aircraft is lower than ideal at high speed and higher at low speed.
For flying wings, sweep will let the aircraft center pitch up and down when the wing flexes. This creates a powerful interaction between the fast period mode (which is only moderately damped in flying wings) with the wing bending mode, resulting in flutter.

In short, when having a choice, the clever airplane designer avoids sweep whenever he/she can. But sweeping a wing back is still better than forward sweep.
Low Speed Characteristics
An airfoil initially accelerates the air which flows over its top surface and decelerates it again over its rear part. On swept wings, this acceleration-deceleration only affects the orthogonal speed component, so the speed component in span direction remains unaffected. This is the reason for the higher Mach capability of swept wings, but also causes the air to flow first inwards and then outwards while transversing the wing's upper surface.
On top, friction decelerates the air flowing around a body, such that a layer of decelerated air surrounds each surface of an airplane. The thickness of this boundary layer increases with flow length, and on a swept wing this friction will initially mostly affect the orthogonal flow component. At around mid chord you will find air which has been decelerated mostly in its orthogonal speed component (since this component was so high over the forward part) and now will be subject to more deceleration of the orthogonal component, so that only the spanwise component will be left over the rear part of the boundary layer. Now this boundary layer will only flow off in span direction, such that a massive increase of slow, low-energy air will be collecting towards the tips.
A thick boundary layer will cause early flow separation, so when the angle of attack is increased, the flow at the tips of a backward swept wing will separate first. This will cause lift loss, and since the tips are also the rearward part of the wing, will shift the aerodynamic center forward. This in turn will make the aircraft pitch up, which aggravates the stall condition. If the separation happens asymmetrically, the aircraft will roll in addition to pitching up. In case of the F-100, the tail surfaces were too small to stop the pitch-up, so once the aircraft crossed into the stall region, it would uncontrollably pitch up more and stall completely.
Modern swept-back aircraft have an angle-of-attack limiter which will prevent the aircraft from flying into the stall region. Also, wing fences help to keep the spanwise flow in check, and vortex generators help to energize the flow such that the early flow separation at the wing tips is sufficiently delayed to avoid the uncontrollable pitch-up. The F-100 lacked all those remedies.

Wing fences on a MiG-17 (picture source)

Vortex generators on a Boeing 737 wing (picture source)

This is great answer.

Thank you so much. Very informative! May I ask you a question for clarification? So in a sweepback, the airflow actually turns toward the outboard because of a fiction as it travels back toward the trailing edge. Here, is the spanwise component of the airflow not affected by the fiction at all?

@lemonincider: Check your auto spell - I guess you meant to type "friction". Of course friction is affecting all movement, but it affects the orthogonal component more because this gets accelerated over the forward part. More speed = more friction.

Hi Peter, is it correct to assume that the higher the sweep angle, the higher is the risk of accidental spin in case of a stall? I'm referring to the Tu-154M Flight 612 crash which had also a flat spin. Why it spun in the first place? Tu-154 wings had vertical fences, similar to Mig-17, do you think that it’s high sweep angle design played any role here?

@ElectricPilot: The most important factor is a rear center of gravity location. If the higher sweep angle comes with enough washout, a stall should still be symmetrical. But in case of Flight 612 severe turbulence compounded the situation and flying at low speed was an invitation for trouble. Turbulence can then lead to an aerodynamic situation that could not be reached by control inputs in calm air alone. I guess the pilots should had asked all passengere to move forward in the cabin - not easy when you are in a flat spin in a thunderstorm.

That's not a vertex generator. That's a vertex reducer.

@user3528438: The VGs are for stabilising the shock location in cruise. Of course they are vortex generators.

Winglet blocks the vortex to reduce induced drag.

@user3528438: You sound as if there would be only one vortex. Trust me, there are more than you can count around an aircraft.

Wing sweep improves the performance by delaying the shock waves and accompanying aerodynamic drag rise caused by fluid compressibility at high (near sonic) speeds. However, there are some disadvantages associated with the wing sweep like:

Wing sweep reduces the slope of the lift curve and the maximum lift coefficient of the wing. This means that the swept wing aircraft must fly at a higher AOA to achieve maximum lift.

Source: code7700.com

The wing sweep induces spanwise flow along the wing, which means that the air has to travel longer distance over the wing. This means a thicker boundary layer and more chances of flow separation.

Source: boldmethod.com

The spanwise flow on swept wings increases the effective angle of attack of wing segments relative to its neighboring forward segment; the result being the rear segments of the wing stall first (tip stall for rearward swept wings and root stall for forward swept wings). If the resulting nose-up pressure (due to the fact that the lift vector moves forward) is not corrected, the aircraft pitches up further, extending the stalled region, resulting in a vicious cycle, resulting in complete stall of aircraft.

This is the reason for the 'Sabre dance' experienced in F-100 Super Sabre. This is further exacerbated by the already high angles of attack during takeoff and landing, as already noted.

Source: code7700.com

Wing bending moments cause torsion in case of swept wings. This requires strengthening of wings, or a complicated design. This affects the forward swept wings to a larger degree than the rearward swept wings , and is one of the main reasons the forward swept wings are not used much.
One important consideration due to wing sweep is the high speed aileron reversal. In case of streamwise angles of twist resulting from wing bending and aileron pitching moment add together, resulting a large amount of nose down wing twist, which reduces aileron effectiveness. Due to this swept back wings have low aileron reversal speeds (while swept forward wings have higher values). This problem is not experienced in wings with constant sweep.

This is also a great answer.


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What is the voltage and amps on the transformer rectifier?
On the Airbus A320 ECAM electrical page there are two TR or transformer rectifier and they convert AC or alternating current electrical power into DC or direct current electrical power. But on the ECAM it shows that TR has Volts and Amps but what are they? Are they the new DC current coming out of the TR or the AC current entering the TR? On the subject of Electrics, what even is AC/DC BATBUS and what is there purpose?

No under the TR is V/A but does the TR have their own voltage and amps or is it the DC V/A leaving the TR or the AC V/A entering the TR?

The TR voltage is the voltage being supplied by the TR. The amperage shown is the demand on the TR.
A bus, be it AC or DC, is a source of power distribution. Typically, a generator will provide power to an AC bus. That AC bus then provides power to a large host of devices which require (typically) 115 VAC. Usually there is redundancy such that if one generator fails, another generator can pick up the bus.
A DC bus usually uses a T-R to convert 115VAC to 28VDC and then distributes that 28VDC power to whatever devices require the use of DC. The same type of redundancies as the AC bus can be assumed.
Aircraft batteries are also another source of 28VDC power. Usually that power is directly fed from the batteries to whatever devices require it. These are often named "Battery Direct Bus," "Hot Battery Bus," etc... They often power emergency systems like engine fire bottle squibs, emergency lights, etc...

What is displayed would be the OUTPUT of the TR.  Those values tell you that (if) the TR is working, and how hard it is working.  If it's supposed to be putting out 12 volts but the actual output is something significantly different, then the TR isn't working correctly.  The amps value tells you how much DC current is being demanded of the TR -- maybe something on the associated bus is consuming too much current (and is thus perhaps the source of smoke that you're trying to eliminate), or another TR in parallel has failed.  The failed TR would show 0 volts & 0 amps, while the operative TR would show normal volts & higher than normal amps.
If you want to know the voltage of the input to the TR, you'd select the generator or bus that is powering the TR to see that.  Voltage is typically a value of a bus, while amps is generally the value of a source, such as a generator, battery, or TR.  The source is putting out however many amps. While you can measure the voltage of a bus coming directly off of an electrical source (so we talk about a generator's or battery's or TR's voltage), you can also measure (and will frequently care more about) the voltage of a bus, independent of what source is powering it right now.  If my Essential DC bus has normal voltage, then the important things on it can be expected to continue operating, regardless of whether the current source for that bus is the #1 TR or the #2 + #3 TR's in parallel or the battery charger or the main battery or the standby battery or whatever else that particular aircraft may have.

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sort: votes


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Were stub wings on early flying boats designed or able to exploit ground-effect lift?
I came across a link to the Dornier Do X, and the Boeing 314 Clipper in another question, both aircraft have similar short wings/floats low on the fuselage, described in one wikipedia article as "stub wings"
Were these stub wings designed to exploit ground effect to aid take of and landing (or could that have been an unintended advantage)? Or is the ground effect of these small wings likely to be irrelevant relative to the full size wings.

The wikipedia articles linked imply their main function was for stability in the water and use as landing stages for embarkation. But they seem reminiscent of the wings of some ground effect aircraft or ekranoplans such as the A-90 Orlyonok

In the early 1920s ground effect was not as well studied as today, so it was not considered during development. These stub wings, mostly called sponsons, were meant for roll stabilization on the water, so they were aerodynamically shaped buoyancy aides. Dornier had studied the optimum hull shape in the early 1920s with water tunnel tests, and the result was the flat-bottomed Dornier hull with sponsons.
The sponsons were mounted with significant incidence, not only to provide some more lift in flight, but mainly to reduce spray when sailing through choppy water at high speed.
From a 1925 Russian memorandum on hydroaviation, about the Dornier Wal, a 1922 flying boat design using sponsons:

The distinctive feature of this aircraft was two protrusions on each
  side of the hull (so-called "gills") instead of under-wing floats
  providing lateral stability. Thanks to its flat bottom, the airplane
  could if necessary take off from snow or ice.

Other manufacturers of that time used outriggers, called under-wing floats in the citation above. One example is the Avro 510, which also used an extra float below the tail for stabilization on the water:

Avro 510 (picture source)
Those and especially their bracing struts and wires produced much more drag than sponsons. Later designs sported retractable outriggers which proved to be the least draggy solution.

Dornier 26 with retractable outriggers at mid-span (picture source)
Ground effect grows with a diminishing ratio of both wing chord over height and wing span over height. The first effect is much more pronounced for sponsons in the most drag-intensive phase of take-off from water, namely when the flying boat changes from buoyancy to planing lift, the so-called hump. The second part of ground effect, however, means that the wings also contribute some drag relief; something that works for all flying boat designs. Yes, there is a tiny and unintended advantage from the ground effect of sponsons, but given that water is 800 times more dense than air and slow planing will involve the full width of the fuselage, any aerodynamic help from the sponsons can be neglected.

Great answer, thanks. Do you think we need a flying-boat tag? (I don't have the rep to create one).

@AndrewM: We have a seaplane tag already; I took the liberty of adding it to your question. Will that be all right?

yup that'll do nicely, thanks :)

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Is there anything wrong with this more symmetric aircraft design, and why isn't it used?
I imagine a fixed wing airplane design that's a lot more symmetric than what I see today. This symmetric design has wings in the middle of the fuselage (neither high wing nor low wing), and the empennage is a symmetric cross. Two vertical fins, one on top and one on bottom, and two horizontal stabilizers. The tail fuselage would taper symmetrically too, instead of the underslope with flat top we usually see today. This would make the fuselage closer to a Sears–Haack body, which I believe is ideal.
In other words, it would look a lot like the XB-42 Mixmaster:

Ignore the weird engine. AFAIK, this design could use an engine anywhere. BTW, this is the only aircraft I know with the middle-wing and symmetric cross tail design—whether experimental or production aircraft.)
The reason I imagine this is so the control surfaces are much more aligned with the center of mass. So the rudders, elevators, and ailerons will not produce any adverse torques when used. I think this will be more efficient, especially when using trims, and also is a simpler design as far as I can tell.
To get around the problem of tail strikes, I imagine a single tail wheel attached underneath it. For small aircraft it might not need to retract. For larger aircraft, I don't know how or where it would retract to, but maybe instead some kind of aerodynamic shield can enclose/open around it, like a prolate or oblate spheroid shape.
(An alternate symmetric design would be an X-shaped empennage—same shape, just rotated 45 degrees. I think for a given yaw, the combination of control surfaces would produce more drag though, but it's a way to increase ground clearance.)
(A more exotic alternate might be to forget about takeoff rotation entirely, and just increase takeoff speed and the length required to literally lift off without pitching up the nose. This might not be feasible for large commercial aircraft though.)
(But maybe best is to add canards with their own elevators, or all-moving canards, which will push up the nose and pivot at the wheel.)
To me this symmetric design seems simpler and versatile. By versatile I mean it looks like it could be used for small and large aircraft, whether prop-engine or jet engine, military or civilian, and not really mattering where the engines are located either.
Is there anything wrong with this design? Why is it not used more often? In fact I do not know of a single production aircraft with this design.
If anyone knows of any aircraft in this configuration, whether experimental or production, please provide a link so I can read more about it and maybe discover some reasons for its success or failure.

This is how a missile is designed.

If the engine is behind the wings in that design, it'll be very interesting how to make a level flight with that ;-)

You are right, a symmetric layout would reduce flight mechanical complications like rudder movements causing a rolling moment on top of the intended yawing moment. But would it be worth it? Airplanes usually fly straight courses and keep manoeuvring down to a minimum. The autopilot will deal with the cross couplings just fine, and aerodynamically the added control surface deflections are negligible.
But the new complications coming with the ventral fin are harder to ignore. As you said, rotation at take-off will be limited and you need to take off B-52 style. Note that the B-52 needs a high wing already to prevent its wingtips from scraping on the runway when there is too little lift to bend them up (beefing up the outrigger wheels would also be an option, but adds weight). Now the aircraft will climb with a nose-down attitude which increases drag slightly. Only the protection for the tail-mounted propeller afforded by the ventral tail justified its existence in the XB-42 (look at the Do-335 for another example). When the XB-42 became the jet-powered XB-43, the ventral fin was removed.
Or how should you load and unload large cargo if not with a rear ramp? The ventral tail would make access to this ramp impossible. Loading from the front needs a stronger and heavier nose wheel or additional struts like on the An-124. Woe betide the crew who forgets to lower the strut before unloading the main battle tank through the front doors. This complication never arises with a rear ramp.
Now for the fuselage shape: The symmetric tube is not as bad as the current upswept tail, but not ideal. Look at gliders: Here the fuselage follows the upwash ahead and the downwash aft of the wing to create the least drag. But again this will restrict the possible rotation angle at take-off. Especially with wing sweep you are better off with an upswept rear fuselage and a shorter runway. Just think of all the resistance in the population if you need to double the runway length at all major airports. Or do you want to limit the utility of your new design by restricting it to Edwards AFB?
Symmetry is only worth it if the aircraft is designed for aerobatics. Even then, a symmetric wing airfoil is much more important than a symmetric vertical stabilizer.

Do-335 has symmetric empannage, but the wing is low, not middle. And yeah, I was imagining a forward ramp for loading cargo. I think a solution to takeoff rotation is to add canards with their own elevators, to push the nose up and pivot at the tail wheel. The symmetric tube is not as bad as the current upswept tail, but not ideal. How are you measuring "ideal" here? I was under the impression that the fuselage with the least drag is a sears-hack body. Are you thinking of some lifting body design, like a tadpole-shape or camber to the body? I'll make some edits.

I know where the wing on the Do-335 was, thank you. I was referring to the tail. The Sears-Haack body is the optimum lengthwise thickness distribution in supersonic flow; here I thought of a fuselage with the least local inclination and, consequently, the least crossflow and separation. That would indeed be ideal. Yes, it would have camber. Rotating around the tail wheel only works when the cg is close to the tail wheel, too.

What is "local inclination" and "least crossflow"? And are you sure about that last remark? Old-school WW2 planes had two main gear under the wings and one tail wheel, and had engines right in the nose, so i imagine the cg is nowhere near the tail wheel yet they apparently could takeoff without much trouble.

@DrZ214: Local inclination means angle of the surface versus the local direction of flow were the body not there. Crossflow is the result of too much inclination: Flow going across and not along the body. And I am very sure about the last sentence. A taildragger does not rotate around the tailwheel. Rotation is always around the main wheels.

@DrZ214, note that the tail-draggers sit on the ground with nose very high, higher than the take-off attitude. That way they lift the tail first during take-off roll and then lower it back a bit, but not all the way to the ground, to rotate. Some unusual examples sit at exactly the take-off attitude and lift off from all three wheels at once. But the main wheels never lift off first.

The biggest single problem with this is the location of the main spar - across the middle of the fuselage. This might not be a problem for single seat military or sports aircraft but would be a major headache for almost any other design. Imagine trying to load 100+ passengers with 50% of them having to clamber over or under a major obstruction in the cabin.
Yes, you can engineer around this but the resultant solution is likely to be heavier and more complex than simply moving the wing up or down and out of the way.

I didn't think of main spar problems. Isn't it possible to have "cantilever wings" that attach to a main rib, but don't penetrate through the cabin? I may open a new question about that.

@DrZ214: Airsick is right, you need to carry the bending moment from one wing to the other, unless you want a folding wing design even before take-off. In jets like the F-104 (mid-wing!) the spar ended in several beefy fuselage frames so the intake and engine could be placed where the spar usually would be.

You could also add the additional headache of the tail scraping at rotation during takeoff

Couldn't you lower the main wing just a bit, into the cabin floor? FWIW, for instance B777 has the floor almost in the middle of the cross-section...

You can do anything, but all solutions other than a straight beam through the fuselage introduce large bending moments and a heavy structure - for which advantage? On ground the wings hang off the fuselage, in the air the fuselage hangs off of the wings. Easy to snap off at the fuselage if not going straight through.

Just to add a very basic consideration to the already good answers here.
While there may be minor advantages of having the vertical tail symmetry, there is no actual symmetry in the vertical plane for any airplane, simply because any airplane needs to produce lift - which is the biggest aerodynamic force. The 'ideal' Sears–Haack body is ideal only for drag, without considering the need for lift.
This makes the airflow non-symmetric in vertical plane, and thus the optimal body will not be symmetric (even from the purely aerodynamic standpoint, before we consider spars etc.).
A high wing, for example, is aerodynamically the most efficient in most cases. It has positive interference with fuselage and will often have less drag than the equivalent middle wing.
Even on XB-42 you may notice that the horizontal tail is placed high, above the wing. This is usually done to reduce the effects of the downwash from the wing.

I thought horizontal stabilizers were usually a bit higher than the wings in order to get "clean airflow" and prevent deep stall. Maybe this is the same thing? The only way I know to avoid this is to make the wing non-lifting (no camber) for the first, say, 2 meters out from the fuselage, and the horizontal stabilizers extend the same distance. Obviously that's a bit of an exotic solution and probably not worth it.

The airflow is never quite "clean" for the tail. There are quite a number of effects associated with the wing downwash: reduced local AoA (which varies with lift); then delay of this variation (which has a small damping effect); reduction of airspeed; etc. Only perhaps a T tail with a low wing is more or less "clean" in normal flight, but this is exactly the configuration that may suffer from "deep stall" - it becomes "unclean" at the worst moment. With some cleverness the wing/tail interference can be used to a benefit (e.g. An-124), but this works in a limited range of flight regimes.

You're right that there aren't any production aircraft with a 'symmetric' design. But quite a few experimental aircraft have them, especially the tail sitter VTOL ones- note that even they aren't perfectly symmetrical, showing the issues.

Convair XFV Pogo By No information - US Navy, Public Domain, Link

Lockheed XFV 1 Public Domain, Link
Looking at these aircraft alone, we can see some of the issues we would face in these aircraft.

The main spar would have to pass through the fuselage, which will severely limit the uses of these aircraft- it would be extremely difficult to use them in passenger and cargo carrying roles (Even if it is used, loading is going to be a problem unless you open the cockpit upward).
Having a lower fin would cause problems. Unless you are going to take off vertically (which is rarely preferred), the landing gear size has to be increased as with severe limitation in the rotation angle during takeoff and attendant problems. This will lead to weight issues and runway restrictions.
Then there are maintenance problems- if you have to change the engines (which for symmetry, has to be inside the fuselage), the only way is to remove the bottom tail (kinda like the harrier, whose wing has to be removed).

Any aircraft has to be designed for production and operation. These issues are too difficult to justify the use of symmetrical aircraft except for very specific purposes- like the XB-47 and Do-335 designed for speed and the XFV and XFV 1 designed for vertical take off.
For any other aircraft, the required tradeoffs would be too costly to justify. Why tinker with what's working for something that offers little practical benefits (flight controls are doing fine without symmetry; even if there's a problem, modern autopilots can handle them fine) or just because it looks good on paper?

Ohh yes I know I've seen the Pogo before. Never seen the XFV 1. But of course the Pogo's empannage isn't symmetric, because it's using a delta wing. It looks like the XFV 1 uses a symmetric X tail tho, just like I envisioned! Tail-sitters are a whole nother genre of aircraft which, in my mind, are not truly fixed wing. Nor are they even tilt-rotors. They're just a whole new genre with its own quirks.

@DrZ214 I agree, though i have to add that if you want a really symmetric aircraft, you have to look at a missile or a rocket. I'm pretty sure that's not what you wanted to discuss about.

You're right. I don't wanna examine rockets except possibly to analyze/compare the symmetric shape. I want something that takes off and lands. If the biggest problem is tail strikes or rotation at takeoff, then canards can solve that by pushing the nose up and pivoting the aircraft at its tail wheel.

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text:
What's this sound? - Dogfight between F14 & MiG23
The following video features real-life audio of a CAP "dogfight" between two F14 Tomcats and two Libyan MiG23's in 1989.

(YouTube uploader: TopGunMilitary)
The audio isn't the clearest but between 6.29 and 6.34 during the fight a electronic interference kind of sound can be heard.
I have heard similar noises to this in other videos cockpit videos, what is it? My current suspicion is that it is some kind of "jamming" signal from the F14's as I believe the pilots in the recordings talk about one of the MiG's firing a missile.
Any opinions would be more than welcome
EDIT:
This is another video displaying the same noise at 1.37:

(YouTube uploader: BongHitBrian)

That is the aural conversion of the input of an AIM-9 missile's infrared seeker head.  The missile allows the shooter to operate it in either a caged mode, that is where the seeker head is slaved to the angle and azimuth of the radar antenna, or uncaged mode, where the seeker head can freely seek out and track a bright infrared source like a jet engine on a fighter.
When the AIM-9 missile made its debut, the aural lock on cues were the only means a pilot had to know that the missile had acquired the target.  Today, pilots can often select this engagement mode if they wish to keep their fire control radars off in order to avoid detection.  When the missile seeker head is searching for a target, it makes a mechanical, raspy growling sound.  Once the seeker head acquires a heat source, the tempo of this growl picks up in conjunction with a steady mid to high-pitch tone.  You will often hear pilots acquiring a lock on an enemy aircraft refer to it as "getting a tone"; this is what they are talking about.

That is the signal from the heat seeking missile sensor that confirms the sensor has a good lock on target and the missile can be launched.


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How do airlines handle uncertainty in passenger weight for weight and balance calculations?
When commercial airlines calculate the mass and distribution of passenger load, in the cases where they do not weigh the passengers directly, they use standard weight measures which have been approved by the appropriate aviation authority.
The standard weights can be broken down by passenger type (adult/child), by sex (male/female) and by other factors such as season or business/economy class. To obtain these figures a survey is carried out at an airport and appropriate measures are taken to ensure the sample is representative.
The result is a single value that represents the standard weight for a male/female/child/etc. Whilst the standard weight may have a minimal bias (i.e. a small deviation from the population mean), it does not represent the variance of the observed passenger weights. That is if the sample mean is 90kg with a std deviation of 10, the there is a ~15% chance that a passenger has a weight more than 100kg (assuming normal distribution, for the purpose of this example).
Given that airline operators have to be extremely careful and conservative with their weight management how do they account for this uncertainty in their calculations? Using the example above, if there were a flight with 100 passengers each >100kg, there would be an underestimation of mass by over 1 tonne (relatively small in comparison to total loaded aircraft mass, but still a large figure in itself).
Are accommodations for this variation incorporated into the standard passenger weight (say use mean + x std. dev.) or does this get included in the operational limits that the airline set for their max weight allowances in their various forms, for example reduced Max Zero Fuel Weight (MZFW) or Max Take Off Weight (MTOW)?
Edit: This is a slightly different question to a similar one about how a commercial aircraft has its mass determined. The focus here is more specific in looking at how the standard passenger weights are calculated and what are the assumptions around using that data.

Possible duplicate of How does a commercial airliner measure its weight/mass?

That answer doesn't directly answer the question, as it mentions that standard weights are used, however there is an informative link which details the process for determining standard weights through survey (http://aircraft-support.blogspot.co.uk/2009/09/aircraft-weighing-jar-ops-16.html). Checking through this link it details how many samples are required to get an unbiased sample (by using the std. dev. in the calculation), but does not address how the variation in passenger weight is incorporated into the weight and balance calculations.

Well, for one it would be statistically very unlikely for 100% of the passengers to be above 100kg in your example.  A "normal" variance will have some above and some below, which tend to average out, but that expected variance has to be taken in to consideration in the airline procedures.

A couple of observations. (2) When trucking sports teams around, an airline will use a different average weight than on a normal runs. (2) A 737 has a useful load of around 75,000 lbs. and carries about 140 passengers. That's over 500lbs of useful load per passenger. The passenger weight is not going to be critical in a large jet.

What about operating from countries where corruption is bad? Yesterday flew klm and had an extra piece of luggage. Got offered to pay a lower rate as long as it got in the pocket of ground handling staff. Apparently the guy told me it's a standard practice... I don't think this will be registered in official paperwork. Even though I did refuse to accept it..

@user3344003: The mass (weight) isn't the only consideration. How the mass is distributed influences the center of gravity location, hence the efficiency of the controls, and the stability.

@mins Obviously.

They don't handle that uncertainty.
Safety is always the prominent factor in the aviation industry. That said, measurements need only to be "accurate enough" for the purpose of safety. Being off by small amount would not compromise safety.
Furthermore, the atmospheric condition is always different; it is so dynamic and cannot be precisely measured. As long as the measurement is accurate enough, the aircraft will fly.
Theoretically, you could argue that there's a chance:

calculated weight is already at MTOW
every passenger happens to weight way above average
they all bought a lot of souvenir at the airport shops
density altitude was extraordinary high
a 10 knots tailwind was present at takeoff

and the aircraft overrun the runway during its takeoff roll. However, such a chance is so small that we accept our current method as it is and expect the aircraft to tolerate that uncertainly within its designed safety factors.
Such believe was found to be wrong when a small aircraft crashed on takeoff in 2003. Regulations were then modified so that on a small enough aircraft, the actual weight of each passenger (plus luggage) must be used, rather than using a standard weight for estimation.

Air Midwest 5841 crashed due to a combination of a) being overloaded and out of balance and b) having had its elevator cable turnbuckles improperly set during its most recent maintenance visit, leaving the pilots with insufficient longitudinal control.  Neither would have been fatal on its own - the plane being overloaded and out of balance was necessary for that crash to happen, but not, on its own, sufficient.

Not only does the FAA define standard passenger weights they have a lengthy AC describing how you should calculate loading. You can find that AC here.

4. What should an operator consider while reading this AC?
a. Accurately calculating an aircraft’s weight and CG before flight is
  essential to comply with the certification limits established for the
  aircraft. These limits include both weight and CG limits. By complying
  with these limits and operating under the procedures established by
  the manufacturer, an operator is able to meet the weight and balance
  requirements specified in the aircraft flight manual (AFM). Typically,
  an operator calculates takeoff weight by adding the operational empty
  weight (OEW) of the aircraft, the weight of the passenger, cargo
  payload, and the weight of fuel. The objective is to calculate the
  takeoff weight and CG of an aircraft as accurately as possible.
b. When using average weights for passengers and bags, the operator must
  be vigilant to ensure that the weight and balance control program
  reflects the reality of aircraft loading. The FAA will periodically
  review the guidance in this AC and update this AC if average weights
  of the traveling public should change or if regulatory requirements
  for carry-on bags or personal items should change. Ultimately, the
  operator is responsible for determining if the procedures described in
  this AC are appropriate for use in its type of operation.

You may be particularly interested in CHAPTER 2. METHODS TO DETERMINE THE WEIGHT OF PASSENGERS AND BAGS
The opening of the chapter pretty much covers that deviations may happen

200. What should an operator consider when choosing the appropriate method?
a. For many years, operators of transport category aircraft
  have used average weights for passengers and bags to calculate an
  aircraft’s weight and balance, in accordance with standards and
  recommended practices. This method eliminates many potential sources
  of error associated with accounting for a large number of relatively
  light weights. However, differences between the actual weight of
  passengers and bags and the average weight of passengers and bags can
  occur when using average weights.
b. Statistical probability dictates
  that the smaller the sample size (i.e., cabin size), the more the
  average of the sample will deviate from the average of the larger
  universe. Because of this, the use of standard average passenger
  weights in weight and balance programs for small and medium cabin
  aircraft should be examined in greater detail.
c. The next four
  sections describe four methods available to operators to determine
  passenger and bag weight. They are standard average weights in Section
  2; average weights based on survey results in Section 3; segmented
  weights in Section 4; and actual weights in Section 5. An operator
  should review the following discussion and consult Table 2-1 to
  determine which method or methods are appropriate to its type of
  operation.

I will refrain from quoting the rest of the document but its worth a read on covers the topic well, it should answer the main body of this question.
Also of interest may be the FAA handbook on weight and balance.
From an operations stand point there are things to consider as well. First off airplanes have a CG range, not a single CG point. This range does in fact have some room to play with. The airlines only need to be within this range for TO/LD. Check out the FAA handbook for more on CG ranges. In theory the airline can (and does on occasion) move people around, although my understanding is that this happens more so in smaller commercial planes. When I fly the Archer or Warrior I typically fly you better believe all my passengers are stepping on a scale before the flight!
Some planes may have ways to mitigate this to an extent. Depending on airframes the fuel loading can be distributed differently among the tanks but you still must be with in limits for TO/LD of course. Any large plane that may also be carrying cargo (for shipping reasons not the baggage its self) may have some ability to rearrange the pallets to keep the CG within limits but this again depends on the situation.
-- EDIT (in response to CG range question in the comments below)--
As for how the planes are designed to handle this there is a CG range. This varies from plane to plane and some have bigger ranges than others. A full break down on how that works could be the subject of its own question here. Generally speaking there is a Max Takeoff weight that must fall with in a CG envelope. Here is the CG envelope for a Piper Warrior

(source)
The charts for airliners have bigger numbers and some more boxes but its easier to describe the issue with a chart like this. As you can see at max gross weight you have 6 inches of CG play. While this may not seem like a lot its more than enough to play with when loading the plane. This leaves room to have passengers of different weights in different seats etc. For the sake of a complete answer it should be noted that these ranges are just the tested limits of the airframe if your CG falls out of these ranges the plane may still fly however it will exhibit different behaviors (high/lower cruise speed, easier entry to a stall or tougher exit etc.). There have been various accidents in history that happened as the result of a massive CG shift in flight, there is some debate as to the role that the over loading of AF4590 (the Concorde that crashed) played in the accident. You should be able to find similar CG range charts for most modern airliners out there if you want more specific ranges.

A very informative answer! I read through the links and this is what I have got from them: 1) there are various considerations that need to be taken into account in order to reduce the survey bias. 2) If the pilot believes there is a significant deviation, then certain measures should be applied - weighing the passengers, or use of pre-approved non-standard weights for certain passengers. However, there is no mention about handling the final uncertainty in total passenger weight, which makes me wonder whether if this is not significant enough to be of concern in practice?

I think its just statistically unlikely that the airplane ends up full of 200LB Summer Males all sitting in the rear, and a bunch of unattended minors in first class. It just not really feasible to weigh everyone, since the plane has a CG range it can have a mass range that moves around a bit as well.

A very low probability example to be sure. The tolerances of the ranges is what I am trying to figure out - how these CG or mass ranges are designed to account for this, or not as the case may be!

The answer to that is long ill add it to the body of my response.


text:
Is there a convention for type of switch used and where?
In most cockpits, you'll find the full gamut of switch types in use. Toggle, rocker, push-button and more. Each has their own "feel" and might be used for different situations.
Is there any convention for what type of switch is used in a cockpit, and where/for what?
Such as, is the Generator usually/always a toggle switch, and would it be "weird" or unexpected for it be a rocker switch instead? Everything could be a toggle switch, so why do manufacturers choose to use different types of switches for different things?
The 737's overhead panel uses toggle, rocker, push-button, and rotary dial all together:

These images are from a simulator, but the aircraft is modeled after the TBM 850. In it, you'll notice toggle, rocker and push-button switches:

Same thing up top in the same aircraft:

Here's a real image showing the same thing:

Are you asking if there is a specific regulation or design standard for what type of switch needs to be used for a certain function?

@RonBeyer A convention that some/most/all aircraft builders would follow. Like, is Generator always a toggle and would be "weird" if it were a rocker or something?

Flaps and landing gear controls are pretty standardized, but I don't think you're asking about them.

I wonder if this paying document isn't relevant: SAE ARP 4102, Flight Deck Panels, Controls, and Displays. Some information in 14 CFR 23.1367 - Switches, and 14 CFR 23.777 - Cockpit controls

@mins Seems to be on the right track, although it doesn't mention types of switches or colors, etc.

I can think of a few instances where a change of the switch between version contributed to pilot mistakes.

Air Disasters tv show - 737 crashes in Panama due to ADI gyro failure. The pilots tried to "make the ADI look right" but the gyro was intermittently failing. The gyro source selector is different from the simulator the pilots train in: "switch" vs "selector knob" - but in both cases the switch is turned/thrown left.  And the gyro source selected is different. So how does "similar but different" factor in especially during a high-stress situation? And clearly standardized switch(ology) can kill you if it helps you forget about what's going on behind the switch.

In C-130B models there was a "killer switch" which caused many a check ride failure, especially for those used to newer models. Selecting a nav-aid exactly like other C130 models did not feed that info to your instruments. You had to throw "the killer switch" to select for instrument display. And of course the killer was on the center console out of the cross-check view of both pilots.

A gambit is a tactic where you deceive your opponent with bait or misdirection and then attack from the sides or elsewhere. A gamut is the full range of something. It comes from an early musical scale where the lowest note was called gamma and the highest called ut.

It seems like a rotary switch would be used for something with more than 2 (or 3) selections, or an analog gradient of some setting (like a heater). Doesn't really address switches/rockers/toggle switches, though.

@radarbob A causative factor in the Panama 737 crash was that the gyro selector toggle switch operated differently than the gyro selector rotary switch: They did not both do the same thing when throw/turned to the left.

This is an excellent question! Some things are obvious - Window Heat is on or off, so a simple toggle makes sense; but then, so does a rocker; but wait, a 2-position rotary will do quite nicely as well, thank you; a push-button will work; as will a momentary contact. Wait, I know, we'll use a touch screen! Yeah, I can see where having a standard convention, especially in the GA world where pilots are more likely to switch planes, would make a lot of sense.

I can't give you a 100% correct answer, but I'll try as best as I can, having some experience with much smaller aircraft (even than the TBM850):
As far as I know, there are no strict conventions for this and this is why you see most aircraft types equipped with their own panels and cockpit design.
You always have to keep in mind that aircraft manufacturers put most of their engineering efforts into designing aircraft concepts for a specific market and then casting their ideas into an actual airframe that fits all the systems that are needed to operate the aircraft.
Allmost everything else (engines, landing gear, avionics, A/C, lights, APU, steering, braking and anti-skid systems etc.) is developed and manufactured by external suppliers with the aircraft manufacturer responsible for adapting and orchestrating all the systems.
This might lead to the aircraft manufacturer simply using already available switch assemblies or only slightly modifying the suppliers' reference designs for panels in the actual aircraft.
IMHO, this is easily recognizable when looking at the cockpits of (especially older) aircraft where there was no concept of the cockpit as a whole. Whatever switch assembly was needed was just somehow installed into the panels.
As Boeing's 707 was a great success, its cockpit design (at least partly) found its way into the 707s successors/derivates/siblings 727 and 737. This lead to the 707 setting the cockpit design paradigms for following (Boeing) aircraft.
Also, from a pilot's point of view, there are some semantic aspects about choosing a switch that 'feels right'. For example:

push-buttons to start an automated process
korry-type switches for switching automatic systems on or off or for on-off switches that might be controlled automatically (And therefor show their status; Airbus uses these a lot due to their cockpit philosophy)
toggle switches for everything that needs an on-off type switch on low-level without other systems needing access to it (for critical switches with safety cap as seen in the 737 overhead panel)
rotary switches for mode selectors
potentiometers for everything that is finely controlled by the pilot
rockers maybe for mode selection between two modes where neither of both is just "off"

are very intuitive to me. This might be training or just experiences from everyday life, but I understand what they are doing even before having read the manuals thoroughly, which makes it easier to remember which switch does what.

Is there a convention for type of switch used and where?

Not in any strictly followed cross-industry way.

Brand X vs Brand Y airliner
Even if two aircraft both use a similar toggle switch for something, they can disagree about which direction is on or off

Note also that brand Y has different toggle shapes for three-position switches but brand X doesn't.
As jxe's answer says, within a single cockpit there is a lot more consistency.

Your bottom two images illustrate a problem I encountered while flying for 2 747-100/200 carriers using aircraft from other airlines. For purposes of illustration, let's say the left image came from the overhead panel of any carrier except TWA and that the right image came from the overhead panel of a TWA 747. The overhead panel was angled, but TWA chose to think of it as a vertical pane insofar as the switches while all other carriers chose to think of it as a horizontal panel. Thus, for the left image the beacon switch is 'forward' for on while on the right the beacon is 'up' for on.

As @Terry states, Boeing offered airlines the special option to flip the switches "upside down". Lufthansa came up with this idea allegedly, and TWA followed. The TWA birds got flipped back when AA bought them, but AFAIK LH still has them today. Otherwise the convention is consistently aft for on.

Check document CS-25 (and CS-23) for "certification specifications, including airworthiness codes and acceptable means of compliance, for large aeroplanes" .
It details all requirements for required indicators, switches and CRTs on large commercial aircraft (23 for not-so-large-aircraft), including the shape of the landing gear lever (must resemble a wheel) and flaps lever (must resemble a flaps),etc.
Goto 'Subpart F - Equipment' and 'Book 2 - Subpart F'.  Also check 'Book 2 - General AMCs' for more detailed requirments on the EFISs


text:
What is the exhaust port on the tail of jetliners for?
Jetliners usually have their engines mounted on the wings, so what is the exhaust port on the tail for?

This is called the apu unit on an airplane. It uses jet fuel to make electricity to power the screen on the back of the seat or the cabin lights. It can also be referred as a tailcone.

That's the exhaust port for the APU (Auxillary Power Unit).
The APU is usually used to power on-board electronics when at the gate, or used to start the main engines. They may or may not be running while in-flight.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
What type of computers are in a flight system?
I know there are many types of flight computer systems, with varying names and capabilities.
I'm more interested in knowing what the computers, themselves, are made of?
They certainly are not "off the shelf" computer components - and are more likely specifically built single-board computer systems running some sort of Real-Time OS. Where can I find out more information about one (and/or purchase one) if I were inclined to test software on one at home?

What do you mean with "flight system"? A generic airplane? It seems a question not very specific.

@GianniAlessandro I assume most have similarities in hardware (underneath the display and buttons). They almost surely don't have an ASUS motherboard and and Intel SSD "under the hood" - I'm asking what hardware is generally used to create these systems.

This question seems very broad, not only are there different computers in different systems, but also different systems in different aircraft.

@fooot This question has been answered satisfactorily. See below. No need to attempt to close.

There are many high level specs available online on manufacturer sites, e.g. this CMA-4000 using a 500 MHz PowerPC cpu. Search e.g. "flight management computer specifications" or similar terms.

Historically, avionics manufacturers have used off-the-shelf processors on custom circuit cards.  Processor selection has been based on getting a reliable processor (established history) at a reasonable price and an expectation that it will be available for a long time.  The high turnover (obsolescence) in processor design along with a lack of design integrity in commercial products is causing avionics companies to move towards custom chips.  Though some products meet the integrity by using two dissimilar processors (though it doesn't address the obsolescence issue).  I am aware of a number of older designs using 486 and PowerPC processors.
Operating systems (just like all the a/c software) have to comply with the appropriate level of DO-178 (currently rev C).  In practical terms, that means the OS has to meet DO-178 Level A.  There are a number of commercial solutions available.  LynxOS-178 is one.  Integrity-178 is available from Green Hills.  Wind River also has a product available.

Exactly the information I was looking for. Thank you.

This book seems to have lots of information on hardware and software (realtime OS VxWorks) used on the 787.


text:
Why not have Pitot Heat always on?
Most (all?) aircraft have a Pitot Heat Switch which must be turned on in certain conditions.
Seems Pitot Heat is one of those things you don't want to forget to switch on when it's needed, or worse, not recognize you need it on and take action based on false readings.
The pre-flight checklists I've read always say something like:
Pitot Heat - As Required

Why not always have it on? Or not have a switch and it just comes on with the aircraft's other power systems?
Is there a downside to it being always on?

related: Why turn off pitot tube heating?

@DanPichelman  That question talks about somewhat the same thing, but never really gives a definitive answer.  On an airliner, we have the system on from the time we push back until shortly before approaching the jetbridge (don't want to risk fabric from the curtain on the jetbridge melting onto a hot pitot tube), so WE essentially always have it on. For aircraft that operate otherwise, I'd be interested to know definitive reasons why.

I agree, the linked question is the same, but apparently none of the answers exactly address it.

@kevin - true, but SportsRacer's answer looks like it may come close - "On the ground, without airflow over the pitot tube to cool it, it will burn out very quickly. By operating something like this when it's not needed you risk not having it available when you need it."

@DanPichelman I've seen pitot tubes that are hot enough to change the metal's color on a B747 during maintenance. The engineer told us they were testing it, so apparently they don't burn out very quickly, as least the ones on airliners.

Not to mention, a simple 30 cent temperature diode could measure temperature of the element, allowing the system to cycle heat on/off automatically, ensuring it never overheats.

@SnakeDoc: But remember that a lot of GA planes where built when diodes were a kind of vacuum tube.  And personally, I don't intend to be flying in conditions where it might be needed :-)

@Ralph J - I think one thing we often forget about with questions like this are human factors. Even if we assume there is some sort of perfectly ideal pitot probe that won't oxidize, won't fail after so many hours, etc... There is still the issue that people will tend to just leave the switch "ON" all the time. After all, the question is asking, "Why not always have it on?" This could cause serious injury to personnel who inadvertently touch the pitot probe on the ground, and it could also cause damage to the probe if you attempt to put pitot covers on it. That's at least one reason why.

@SnakeDoc - that's one more component to fail, which could disable the heating element

I can generally only speak for commercial aviation. The reasoning is probably the same or similar for General Aviation, but seeing as that is such a wide and varied field, with a tremendous number of different aircraft, probe types, etc., someone somewhere could probably find some niche corner-case to dispute the reasoning. My guess would be that, generally speaking, in General Aviation, certain probes are most likely only meant to run for a certain period of time. After all, it's not like a Piper Cub is going on an 18-hour non-stop flight to China like a Boeing 777 might. I'm not saying that a probe will fail after 18 hours of being on, just that repetitive use in that way might necessitate a different type of probe with a more robust design.
In commercial aviation, many modern airliners use certain logic to turn pitot heat on or off. With the exception of a manual override switch equipped on some aircraft, to force it off, the pitot heat is generally on during most flight phases and works as such:
On the ground, with engines off, the pitot heat is turned off. On the ground, with engines running, pitot heat is on low. Finally, once the plane takes off, the pitot probes are put into a "high heat" condition and remain that way for the remainder of the flight (unless manually turned off).
The pitot heat is generally monitored with current sensing relays such that if the pitot heat draws excessive current or not enough current, the crew will be alerted to a pitot heat failure.
Why not keep them full hot all the time? For one, that would most likely cause premature failure of the heating element. Another reason is that, on the ground, pitot covers are normally applied to prevent bugs from building homes inside the tubes as well as preventing other debris from entering. The pitot covers will melt and destroy the pitot probes if the covers are left on while heat is applied. This can also be problematic and dangerous if the pitot cover partially melted, wasn't noticed by ground crews, and the drain holes or entry holes were blocked by the melted cover. Finally, and probably most importantly, the pitot probes get extraordinarily hot. Speaking from experience, and from having touched one that had failed in the "full hot" phase, it takes only a brief second to receive excessive burns on one's hand. Most of us that have been around them in our lifetimes have been burned by them, and a savvy mechanic will always approach them with caution!
So, it makes sense to have the probes off, on the ground, with engines not running. Otherwise, commercial aviation has more or less decided that "full hot" in the air, and low heat on the ground with engines running is a design characteristic.

Depends on the airframe and where it is operating.  For light aircraft operating a low altitudes (<10,000 ft) pitot heat use is going to be dependent upon whether the pilot anticipates encountering ice or freezing conditions.  This should keep the tube unclogged be free of ice should it be encountered - though I have my doubts given how anemic the pitot heat systems on light Cessnas are.
Business jets and airliners are another story altogether.  Designed specifically to operate in the flight levels and hard icing conditions, the anti icing systems on these aircraft are much more capable compared to smaller GA aircraft.  They can safely operate in all but severe icing conditions and are VERY effective compared with their bugsmasher counterparts.  The pitot probes, static ports, air data sensors and AoA vanes get so hot during use it causes oxidation on their surfaces.  Pitot/AoA heat is usually engaged just prior to takeoff and disengaged just after landing to prevent ramp hands from being burned by the probes.

Okay, but what drives the non-continuous operation on the light aircraft? What is gained by leaving the pitot heat off when it's clear & VMC?  Saving electricity?  Prolong the life of the heating element? Risk of burning it out if you forget to turn it off after landing? Something else?

As I interpret such a checklist item, pitot heat is to be used AS REQUIRED for the safe completion of the flight, usually if the pilot anticipates entering clouds, or other areas with moisture where freezing conditions exist and icing might be encountered.  Could you switch on pitot heat for the full duration of the flight?  Sure, but it's unnecessary.

Unnecessary for clear skies, but for airliners we leave it on full-time, regardless. (We only turn ENGINE anti-ice on "as required" when we're entering potential icing conditions -- IMC + low temperatures -- because there is a drawback to running it continuously: loss of power / fuel efficiency.) In many GA aircraft, pitot heat is not run continuously, and the question is, why not? What's lost by leaving it on from takeoff through landing that offsets the (presumed) advantage of having it already on when you need it, no "remembering to turn it on" required?  SOMETHING is different; what it is?

All good responses.   Actual temps of heated probes can be as high as 1200F per our in house tests.  Air data sensors (TAT, ICE, AOA, etc.) all require protective covers at some point for a variety of reasons.  Best not to have heat on when applying and removing protective covers.

If you heat the pitot tube while in flight, it will keep a reasonable temperature. If you heat it when you are on the ground at a normal temperature and without any forced convection, it will reach a temperature of some hundreds degree. This is very bad for three reason.
1) Wasted energy: why to waste energy? It has no reason to waste precious (and expensive) electric energy. It is a reason also to keep it switched off during the flight in all those conditions that do not present ice risk.
2) Corrosion and wear: the higher the temperature, the faster the corrosion process and the wear of the component. The more you use something, the sooner you will have to inspect and replace it.
3) Safety: if you are a technician and you touch the pitot tube on the ground you lose your hand (and I can tell you for sure that it happens sometimes, when you have to perform tests and you turn it on with weight on wheel).


text:
Time to switch instructors? or not?
To give a bit of background before I get into my scenario, I recieved my PPL in Canada two years ago, and then my night rating about 10 months after that. I was able to remain current but due to financial reasons I only just recently was able to go for my CPL.
The instructor I have now was just randomly assigned to me, and took me up to show that I was able to do upper airwork. Needless to say my skills had eroded, which wasn't all that surprising - so it took a few lessons to get all that rust off. Then we decided to practice diversions, which again my skills were eroded.
Now I have been studying all these maneuvers at home and playing them over in my head to get all the steps down pat, which I know isn't the same as flying. Having said that, I am not flying that regularly either which is down to my schedule and things pop up sometimes on my 4 days off so I do take full responsibility for that no doubt.
My instructor isn't all that encouraging though, he basically is trying to tell me without saying it, "stop wasting your money and stop flying". He doesn't try to really help, he basically criticizes me, which I am able to take, and he's not wrong on what he is saying -  but that's it, he doesn't really go much further than that.
He also has mentioned that my walkarounds are taking too long and that I am not fast enough at it, and I should be quicker with checklists, when all I am doing is being methodical and sure that I am not leaving anything out. Another thing is, half the time we don't do a briefing/debrief so the first time we practiced diversions again, we went up without a CPL diversion briefing and then we came back and he basically told me that wasn't up to CPL standard and this is how we do it.
So shouldn't he have done a debrief so I could know that? Another thing is he was late for a lesson by a half hour, but gave me flack for being 5 min late... I do feel frustration with him but I am not one to talk openly about that frustration so maybe I should grow a pair and talk to him about it? Before switching instructors?
Sorry for this being long but thanks to anyone who has tips or criticism.

Welcome to aviation.SE! Because this is a Q&A site, not a discussion forum, we generally don't answer questions that ask for opinions, discussion, suggestions etc. The [tour] may be helpful if you're not familiar with how the site works. You'll probably get a much better response on an actual discussion forum, where you can ask follow-up questions and get multiple points of view.

I'd recommend getting a flight simulator setup at home, so that you can practice a bit before getting back into the plane, and again after each session. It's not 100% the same, but it can help with maneuvers, timing, and familiarity.

How old are you? If you're over, say, fifty, it is to be expected that you may take a longer than otherwise expected. Also, how old is the instructor? That can enter into it as well. And, though it may be impossible to find out, how many hours of instruction has he given. My experience as a former chief flight instructor at a U.S. FBO that did a lot of training, is that it takes a few hundred hours of instructing to really figure out how to tailor your instructing to individual students. In the end remember that, for all practical purposes, he is your employee.

The question is off-topic (opinion based), but the answer seems to be a clear "yes" - switch!

Regarding preflights, I have the same issue as you; I like being thorough. (If I'm a few thousand feet into the air with nothing between me and certain death except for that machine, even though I mostly trust it will get me both up and back down safely, if there's a problem then I sure as * want to catch it on the ground if I can!) What I ended up doing is to book the aircraft for a brief period before the beginning of the booking with the instructor. As long as one is paying only for flight & instructor time, that gives time to give the aircraft a good preflight w/o stressing out over time.

I did not realize this was an non opinion based forum my bad. I am just 26 and he is not much older I dont think, he is a relatively new instructor and this flying school is relatively busy during the summer so he is as fresh as they come

It's your money. Spend it on someone you are comfortable with can have a good rapport with.

It's just you and the instructor up there.
No way would I stay if you are not feeling good about
it.  Would not want to think about how both of you would react in a real emergency.
Nothing wrong with changing the instructor, or even flying schools.  Fresh start somewhere else might be good.
First, I would speak to the boss there.  But rather
tearing the instructor apart, simply explain your situation and get with someone who understands and is
compatible with you.
I went through 3 trainers learning how to drive my
truck.  Now, 4 years later, I can laugh about it a little.
Watch plane crash stories on you tube.  You will notice that the pilots many times either not getting along or not talking at all.  They can't work together.  Can't have that.
Some instructors may be on the bottom rung of the piloting ladder anyways, struggling themselves.
I would play the field and look around a little.
Interview people and visit other airfields.
Might be the best move, particularly considering what
lessons cost. Good luck.
Bob


text:
Identify a balloon accident in the 90s or 2000s
I recall hearing about a balloon accident on a news programme on TV in the 90s or 2000s. From memory it was some sort of record attempt, and the last telemetry from the flight indicated that the gondola was in free-fall.
I seem to recall a crew of at least two people. I think it happened over the sea, certainly far from where anyone could observe it, hence the need to rely on radio for communication with the crew and telemetry.
At the time of the report it wasn't know exactly what happened or the fate of the crew, but the free-fall telemetry made it sound pretty bad.
Can anyone identify this accident?

Any more information? Such as the country it happened in?

@Jamiec yes, it was over the sea I think. I'll add that to the question.

which sea? There is a lot of sea on the planet!

Are you sure you're not thinking of Steve Fossett who successfully circumnavigated the world in a balloon but then disappeared (never to be found) while flying a light aircraft over the nevada desert?

Maybe you mean this accident? It fits every information you gave except the free fall.

@Jamiec You're exaggerating a little.  Fosset's crash site and remains were found in 2008.

@ZeissIkon You're right - that was from memory and my memory was wrong.

@Jamiec definitely wasn't Fossett

@Bianfable not that I think. I've been through Wikipedia's list of balloon accidents and notable balloon aircraft and didn't find it.

I don't think this is answerable without more details.

Another morbid thought - hope you're not confusing this with the challenger accident. Negative for "balloon" but positive for "freefall", "telemetry", "over water", "record" (first teacher in space) and (horribly) "crew"!

@Jamiec maybe... I would have been very young but it's possible that there was something about the investigation after the fact that I saw.

P 15 of this link describes an incident on July 20 1990 where an unmanned scientific balloon project accidentally experienced a freefall of the payload from 120,000'.  Any chance that what you are remembering was this, or something similar?
http://hdl.handle.net/2060/19910006317

The 1990 calibration actually consisted of two flights,
  one on July 20, 1990 and the other on September 6, 1990.
  A malfunction occurred during the first flight, which
  resulted in a complete loss of data and a subsequent free fall
  of the payload from _ 120,000 ft which bent up the tracker
  assembly and destroyed several cell modules. The tracker
  was rebuilt, several cells were replaced, and the refurbished
  payload was flown again in September.

I've edited in the relevant part from pp15 - links (and especially pdf's) tend to move/get removed so it is good to quote the relevant portions of text that constitute your answer.

It was definitely manned, but thanks anyway.


text:
What happens when a takeoff clearance is cancelled by the ATC?
Let's say an commercial passenger jet is accelerating down the runway, taking off. The ATC receives some signal that he/she should cancel all takeoff clearances.

Exactly what happens in the control room?
What does the ATC do and how does he/she make sure pilots understand the cancellation?
How do the pilots in the aircraft taking off respond? Are they trained for this scenario?
Is a takeoff cancelled even after V1?

There can be special cases, so to speak. For example, at Tokyo Narita in the 1990s, freighter flights sometimes pushed the 2300 curfew because they had had to wait for cargo. The tower would clear us for takeoff as we were taxiing. We knew that if we did not begin our takeoff roll by 2259 (they allowed one minute for the takeoff roll of a 747) they would cancel the takeoff clearance. That would occasionally happen as we rounded the corner to line up on the runway, in which case we would not acknowledge their transmission and continue the takeoff. They never complained.

Your quote is misleading. You omitted the part which says "already cleared the next departure". The aircraft whose clearance was cancelled did not commence take-off roll yet (otherwise it's clearance couldn't have been changed to line up and hold).

@JanHudec, could it happen, hypothetically? Because what I was interested in was the communication and procedures surrounding such a time critical event.

@JonasG.Drange: Yes, it does happen sometimes. I'll add references to some other incidents to my answer below.

@JanHudec, I've removed the bad references. Feel free to edit in yours into the question. Thanks.

What happens in the control room?

Aborting a take-off at high speed is dangerous, even if the aircraft is travelling below V1. If the aircraft is already rolling, the controller will make a quick decision whether the situation justifies an aborted take-off. For example, if there is traffic infringing the runway and a crash is imminent the controller would attempt to stop the aircraft. If an aircraft is attempting to take-off from a taxiway in snowy conditions it might be safer to just let him take-off unless there is other traffic on the taxiway.
If the aircraft hasn't started rolling the decision is of course easily made.

What does the ATC do and how does he/she make sure pilots understand
  the cancellation?

Using standard phraseology is the best way of making sure the message comes across, especially in international aviation.
If the aircraft is not yet rolling, the proper ICAO phraseology is:

Tower: Big Jet 345 hold position, Cancel take
  - off, I say again cancel take
  - off (due to vehicle on the runway)
Big Jet: Holding, Big Jet 345

If the aircraft already started rolling the controller would say:

Tower: Big Jet 345 stop immediately, (Big Jet 345 stop immediately )!
Big Jet: Stopping, Big Jet 345

No immediate verbal reply is to be expected as the flight crew have other things to care about at that moment. The effect of their actions should be clearly visible from the tower.

How do the pilots in the aircraft taking off respond? Are they trained for this scenario?

The crew will assess whether they can stop safely. Below V1, they will probably abort the take-off, above probably not. The final judgement is made by the captain. He might reason that it is safer to get airborne and pass the infringing traffic overhead than running into it at full brakes. They are trained for aborted take-offs, but since there are so many non-standard situations the final decision is made on the basis of a mix of training and expert judgement.

Is a takeoff cancelled even after V1?

Since ATC does not know when the aircraft passes V1, a controller might attempt to cancel the takeoff after V1. The flight crew will only attempt to cancel the take-off after V1 if it is clear that there is no way the aircraft can become - and stay - airborne safely. In such a case it becomes damage control rather than damage prevention.

Tower will tell the pilot that take off clearance is revoked and the pilot will hit the brakes hard in the standard abort takeoff procedure unless the plane already passed V1 then he will continue the takeoff while hoping the reason won't hit the plane.
After V1 the plane is committed to taking off because it won't be able to stop in time before the end of the runway.

Thanks. Do you have an example of what the ATC would say, specifically? Would he scream in panic, or be clear and concise?

ATC does not revoke clearance for aircraft that already commenced the take-off roll. They instruct them to "stop immediately". That did not happen in this case.

"After V1 the plane is committed to taking off because it won't be able to stop in time before the end of the runway"
This is is incorrect.
V1 is the speed at which a rejected takeoff can be initiated IN THE EVENT OF AN EMERGENCY. Imagine a perfectly functioning Dash-8 on a 12,000 foot runway. You don't think it would be able to stop at Vr or V2?
V1 is calculated based on Vmcg, the minimum controllable velocity on the ground, based on asymmetric thrust from a failed engine. Above Vmcg, the plane would not be controllable on the ground with one engine out, and so it is committed to flying.

reference:  http://www.airbus.com/fileadmin/media_gallery/files/safety_library_items/AirbusSafetyLib_-FLT_OPS-TOFF_DEP_SEQ07.pdf

@rbp: No, above Vmcg the plane is controllable on the ground with one engine out (Vmcg is the minimum speed at which the aircraft has enough rudder authority to counter the asymmetric thrust from a failed engine), and can continue the takeoff.  Below Vmcg, continuing would lead to a loss of directional control, and the takeoff must be rejected.

Your quote suggests that AM-18 aborted take-off because of ATC instruction, but if you read the article carefully it's not the case.
The AM-18 was accelerating for take-off when the crew noticed something is wrong with the tires (probably blown tire already) and aborted just below V1 due to their own decision.

Exactly what happens in the control room?

I don't know exactly, but there are various sensors designed to warn of possible runway incursions. Apparently the sensors detect aborted take-off and alerted the controller.
In some places, including USA, the controllers will clear aircraft to take-off or land when they expect the runway will be clear by the time the aircraft reaches it. So they have to cancel that clearance when there is an incident and the previous aircraft does not vacate the runway as happened here. In other places, e.g. Europe, they don't do this, so they don't have to cancel any clearances, but the pilots are often receiving their clearances at the last moment which has it's own disadvantages.

What does the ATC do and how does he/she make sure pilots understand the cancellation?

They cancelled clearance of the next flight that was just about to line up behind AM-18. I believe "cancel" is standard wording, so something like "tower, XYZ, take-off clearance cancelled; line up and hold." They know the pilot understood since the pilot reads back the instructions in reply.
If the aircraft has already commenced take off roll (which didn't happen here), time becomes critical, so then the instruction is just "XYZ, stop immediately", often repeated twice, or until the aircraft starts to brake or the pilots answer.
It does happen now and then. See for example this incident.

How does the pilots respond? Are they trained for this scenario?

If the clearance is changed, the pilots simply follow the new instructions.
If "stop immediately" is requested, pilots abort the take off if possible.
If the pilot does see the problem (nobody will request aircraft in take-off roll to stop unless there is a serious risk of collision), which they often do, they may decide to continue the take-off if it's more likely they'll lift off before the obstacle than that they stop before it. Captain is the final authority responsible for safety of the aircraft, so it's their call.

Is a takeoff cancelled even after V1?

No, because the aircraft is no longer able to stop on the remaining runway. Instead the pilot may rotate earlier. The Vr is higher than Vmu, so they can rotate at slightly lower speed and they can increase thrust if they are doing reduced thrust ("flex") take-off, which they now usually do.
For example in this incident ATC instructed the crew to abort (but failed to follow correct phraseology). At the same time the crew noticed the problem themselves, but they were already above V1 and decided they can fly over the obstacle and not stop before it and therefore continued the take-off.
Note that at V1 the aircraft is already long way down the runway, so the obstacle  is likely near the end of the runway and likely can be already seen by the pilot, so the pilot decides how they try to avoid the collision.

you're confusing V1 with ability to stop the aircraft. V1 says nothing about stopping before the end of the runway. V1 is decision speed for an engine failure. You're probably confusing it with balanced field length.

@rbp: V1 is decision speed for an engine failure. It is restricted by two conditions: that the aircraft is able to stop if it did not reach it yet and that the aircraft is able to take off with one engine inoperative if it did reach it. Balanced field length is minimal runway length for which that range is non-empty. When the runway is longer, there is a range of speeds satisfying the condition. But it's always better be on the ground, so the speed gets selected from the high end of the range. i.e. from the end limited by requirement to stop on remaining runway.

@rbp: Actually, there is a difference between GA aircraft and airliners here. GA aircraft usually only have a table for the balanced field, so their V1 is in the middle of the range for longer runways. But for airliners the exact calculation for given runway, altitude and weather is provided and then V1 is selected from the high end of the range. Evidenced by the fact that on long runways usually V1 = Vr.

A takeoff clearance cannot be revoked. Once you issue a clearance the aircraft is CLEARED for takeoff.
If the tower realizes that something that will obstruct or threaten an aircraft taking off is present, all they can do is inform the PIC of the situation. At that point it is up to the PIC to figure out what to do. They might try to abort the takeoff or perform an evasive maneuver.
To clarify you this for you: a tower cannot tell an aircraft to abort a takeoff because they are not qualified to make that determination. The guy in the tower neither is qualified nor can see the instruments of the aircraft so they have absolutely no idea whether it is safe to abort a takeoff or not. Therefore, they will never tell a pilot to abort a takeoff once it is underway. All they can do is cancel the clearance (See Section 9. Departure Procedures and Separation in FAA ATC guidelines). The phraseology is:

[Call sign] Cancel Takeoff Clearance [state reason]

Of course, if the aircraft has reached V1 it will not abort.
In situations I have been in where something bad has happened is that the tower will just tell the pilot what to do. Like if some plane tries to land the wrong way on a runway when you are taking off, the tower will say "[call sign] Bear right immediately, traffic dead ahead!" So, you basically try to veer to the right to avoid the head on collision.
To answer your specific questions:
Exactly what happens in the control room?
In a crisis scenario, the tower guy will say whatever he/she thinks is necessary to avoid a collision. Once I had a near collision because another guy was not where he supposed to be, but I was the one with comm, so she moved me by screaming "TURN LEFT RIGHT NOW !!!" Phraseology goes out the window when you are 0.5 seconds away from a huge fireball with multiple fatalities.
What does the ATC do and how does he/she make sure pilots understand the cancellation?
(I gave the phraseology above.)
How do the pilots in the aircraft taking off respond? Are they trained for this scenario?
If you can safely abort, you abort and slam on the brakes. If not, you keep going and try to maneuver around whatever it is, moose, truck, whatever. Pilots are trained to never abort when they reach V1. Other than that it is experience and nerves that count, no more training.
Is a takeoff cancelled even after V1?
Cancelling is what the tower does. Aborting is what the pilot does. The tower cannot "cancel after V1" because they have no way of knowing if the plane is at V1 or not. A pilot will almost never abort after V1 because the consequences of going 100 miles an hour off the runway are almost always worse than the consequences of going 100 mph in the air. If an unavoidable obstruction occurs, like a plane rolls onto the runway in front of you, and you are at V1 you still takeoff, you just try to do it sooner.

You start off by saying a takeoff clearance cannot be revoked, and then you go on to describe how it can be canceled... I find that a bit confusing.

@fooot True, it is confusing. Personally, I think they should not even have that phraseology there, because as I said the tower is not qualified to judge whether a takeoff is abortable or not. So, I guess, the way to look at is that the tower is free to "cancel" their clearance and the pilots are free to ignore the tower's "cancellation". That's the way things stand now.


text:
Why don't [most] sailplanes use whole-airplane / ballistic parachutes?
My understanding is that sailplanes usually have the pilots wear the parachute on their back, requiring the pilot to bail out of the aircraft before they can use the parachute.
Given the maturity of modern whole-airplane ballistic parachute technology, this seems strange to me given the drawbacks of having to bail out: it takes time, ability to bail can be affected by G forces, presumably requires some parachuting skill, and is not really a solution for unskilled passengers.
There are even airbag solutions like DG Noah that help pilots bail out of the sailplane.
So, why aren't sailplanes equipped with ballistic parachutes, given how popular these systems are becoming on various ultralight aircraft? Does something about the sailplanes' construction or performance parameters make this type of aircraft a poor fit for whole-airplane parachutes?

I wonder what you are trying to fix? A glider can by definition already glide very well, so why would you need a system that makes the aircraft glide towards the ground without any pilot control? The Cirrus planes only have the parachutes for spin recovery.

Just take into consideration at what flight situations critical accidents occur - and what altitudes are needed for a safe deployment of the parachute. Not much of an overlap. On GA-aircraft the accident-envelope is much wider, so it does make much more sense here.

@Bianfable probably 90% of single seater glider pilots wear parachutes.  The midair collision risk is high and in my experience is the biggest risk in soaring.

@JohnK I see. So this parachute might be used after a collision which renders the glider uncontrollable?

@Bianfable yes exactly. At a glider club on a busy afternoon you are on intense traffic alert the entire time. I've had a few close calls over the years.  FLARM, the poor man's TCAS system, is extremely popular in the soaring world. My club has it installed in all club a/c and just about all the private owners have it.  Parachutes tend not to be worn on 2 seat training flights however; the average passenger won't be able to use it properly.I can see a ballistic system being adopted on 2 seaters but not on singles where you normally wear a parachute all the time and the seat is designed for it.

I guess adding a parachute adds responsibility (in case of defective deployment) and certification costs for the manufacturer, while individual parachutes are pilot business. In addition individual parachutes may allow a better control of the landing site, at least for trained users.

Perhaps because you don't bail out of a glider unless it's already seriously broken.  So what would be the point of parachuting the wreckage to the ground with the pilot inside, when it is probably safer for just the pilot to parachute?

I would just as soon leave the a/c with a parachute on my back than ride down in the glider with one. I have witnessed a midair between two gliders right near the field, abt 20 yrs ago, and one guy was saved because the glider disintegrated around him and he had not much choice but release the harness to free himself from the fuselage pod  and pull the ripcord to stop plummeting.The other guy had semi-control of his glider and tried to fly it, but lost control eventually and went straight into the ground and was killed. I'm an ex jumper and  likely would have less inhibition about bailing out.

To answer the why: midair collisions, control linkage failures, medical emergency, running out of energy over inhospitable terrain (cockpit provides extra protection)

@Nikita: Re the cockpit providing extra protection, it seems more likely that it would provide lots more hard surfaces for the pilot to hit, and potentially sharp pointy bits to impale the pilot.  Seems safer, overall, to bail out.

@jamesqf I guess that could be one reason. On some ultralights they use a mix of kevlar and carbon fibre in the cockpit area, such fabric doesn't splinter into sharp bits like pure CF would (also helmets, but there's not much space for those in a glider cockpit).

why aren't sailplanes equipped with ballistic parachutes?

Because of their longevity. Most were built or designed before full-airplane parachutes were a thing. When Ballistic Recovery Systems started, their initial offering was for ultralights only. This makes sense because the limited speed range of ultralights makes the parachute simple.
Higher top speeds mean that the parachute needs to unfold in stages so the forces are limited and deceleration is gradual. This is achieved with first a small extraction chute or a rocket which pulls the main parachutes from their compartment. Those are partially sewn so they unfold only partially first. Next, the stitching is torn open gradually to draw out the full opening of the main chute(s).
Compare that to a conventional parachute which is only opened when the pilot has separated from the glider and has decelerated already enough so the parachute design can be much more simple. The glider cockpits are designed for pilots wearing a chute (see CS 22.785 (c)) and the fuselage is kept as small as possible, so the retroactive installation of a much more bulky full-airplane parachute is not possible. Note that the parachute also needs to be fixed to suitably strong attachment points. This means a complete redesign of the center fuselage. Doing this to an existing glider is simply not worth it.
To incorporate a full-airplane parachute needs the development of a new parachute together with the airplane. This has happened first with the SB-13 experimental flying wing glider in 1988. You cannot simply slap an existing chute to an arbitrary glider. This glider-parachute combination now must be certified against regulations which do not yet cover full-airplane parachutes. Given that glider manufacturers are mostly small, privately held companies, most shy away from the expenses involved.
You may also say that market pull is not strong enough. If everyone would be clamouring for their own full-aircraft parachute, someone would have started to offer new designs with them. So far, the market demand simply isn't there.

Opposing viewpoint: BRS are already used in gliders
The AC-4C and Phoenix both have models with a fuselage mounted BRS. I'm sure others do as well, I'm simply not familiar with them.
I wear a parachute when I fly, but I would much rather have a BRS. It's hardly any heavier but it is much, much more useful, IMHO. A BRS can be triggered from within the safety of the cockpit, and requires much less coordination. It also works from far lower altitudes, and the repack requirements are far less onerous (although repacking a BRS is much more expensive). I also suspect it results in fewer injuries, as there is no chance of getting hurt while exiting the glider and the ground impact is cushioned by the fuselage.
As to why all manufacturers don't offer BRS on all modern planes, I can't give any facts, only guesses (which is not really appropriate for Aviation.SE). For starters, BRS is relatively new on the scene, and due to the extremely small production volumes and tight finances gliders aren't hotbeds of innovation.
It might also have to do with the fact that many planes have water ballast, which has a huge impact on the overall weight. A plane with 500kg of water might need an impractically large parachute.

I've never had occasion to use one so couldn't say! I might not be fully informed about the problem, but I consider rolling as a "not-bad" result. In my mind, it means the plane will decelerate more gently. The parachute will prevent the plane from falling uncontrollably.

I concur that some sailplanes do use BRS, but those are clearly in the minority. Vast majority seem to use pilot-worn parachutes.

@Kenn Sebesta rolling may be far from not bad.  Speaking as a paraglider pilot (a glider technically but far removed from a sailplane) the primary concern after reserve deployment is to stop the main wing from interfering with the parachute.  Downplaning and rotation can result in high descent rates and deflation of the parachute.

@Frog, I wonder if you're talking about a different type of rotation. My response was to Ymb1's question about whether it would be bad to have the wing impact the ground first and then have the cockpit rotate about that wing as it finishes its fall. Since even the largest gliders will only have an 11-12m half wingspan, I think this is a very different case from the one you outline. Correct me if I'm wrong, but in your case you're referring to continuous rotation while failing, right?

@ Kenn Sebesta you’re right.  I can imagine that a sailplane that’s been involved in a mid-air might have a considerable tendency to roll uncontrollably and exiting it might be a good idea.  Agreed, rotation after first contact with the ground is generally a minor concern.

BRS chutes weigh a lot. From a cursory google search, they range from 30 to 100 pounds- 30 pounds is light enough that you could lift it if you’re an average human being, but it’s not insignificant. Glider manufacturers and pilots try to shave every possible bit of weight off the aircraft. While there’s enough risk to wear a parachute, most pilots will not have to bail out of their aircraft at any point in their flying career. Now, in the small group of pilots who have to exit the aircraft, how many of those will be in a situation where they cannot jump out? A whole-airframe parachute would be great for safety, but it’s also another potential point of failure- it could go off unintentionally, but it could also just not deploy when you need it. Pilots already wear parachutes and in most cases can get out. The reason BRS are not used is because there are better options.

Just for reference, for sailplanes BRS weight would be approx 25-30 lbs for a system like this https://www.galaxysky.cz/grs-6-600-sd-speedy-115m2-p20-en plus, as someone else pointed out, the necessary reinforcement to the sailplane's structure to support shock loads.